The automatic switching-on operation for a third brake when actuating the motor-vehicle brake valve by means of a brake pedal or footplate has been proposed heretofore in that a motor brake is switched on by slightly pushing down the brake pedal in a stepless manner. Air is then supplied, via an air-inlet valve, initially to the operating cylinder of the motor brake and, when pushed further down, also to the operating cylinder of the friction brake, i.e. the service brake.
Furthermore, there are known switching arrangements on motor-vehicle brake valves which switch on the third brake in several braking action steps, in which case, particularly as far as omnibuses are concerned, retarders with three switching steps are used. In the case of the switching arrangements known thus far, these three switching or braking steps are controlled, in vehicles with a pressure medium brake system, by means of electric and pressure-medium operated switches, the first two braking steps of the retarder being as a rule switched on by means of electric switches before activation of the friction brake and the third step being switched on by means of pressure-medium operated switches shortly after activation of the friction brake.
However, the control of the three switching steps of the aforementioned actuating device, or of the three switches by means of the stepped piston of the motor-vehicle brake valve, has some disadvantages which are objected to notably by companies in the transport industry. The rubber element which is usually located between the brake pedal and the stepped piston and which serves as a transmission element, leads--when the braking steps are switched back or off the rubber element, compressed by the pressure on the brake pedal, takes on its original shape as a result of the removal of the pressure--to an undesirable path hysteresis which in the third step is even increased on account of the hysteresis of the pressure medium. It has happened at times that the second step switched back while the third step was still switched on due to the addition of the hysteresis caused by the rubber element and the pressure medium.
A further disadvantage proved to be the fact that the switching operation of the first two steps, which takes place in the free path of the stepped piston, i.e. before admission of pressure mediums, proceeds without any detectable mechanical pressure point, so that undetected switching from the first to the second step produces undesired sharper braking action, which may sometimes have unpleasant consequences for the following traffic and for the passengers standing in the bus.